To improve traction (grip) of footwear such as walking boots, running shoes, football boots etc., the soles commonly have a plurality of studs (sometimes referred to as cleats) extending from the bottom surface of the sole. The studs are normally spaced apart from one another.
When the wearer of the sole walks or runs etc., upon ground contact, the studs are designed to penetrate or otherwise interact with the ground, so as to inhibit sliding of the footwear over the ground. As the studs contact the ground, a force is applied to the studs in a direction normal to the bottom surface of the shoe sole, counteracting the wearer's weight, and also in shear directions, i.e. in a direction substantially parallel to the bottom surface of the sole. The force applied in the shear direction may be, effectively, a ‘braking force’ or ‘accelerating force’, which inhibits or effects, respectively, further movement of the studs with respect to the ground.
However, with this conventional stud arrangement, the studs have a propensity to pivot about the connection point between the stud and the sole. This effect is exemplified in FIGS. 1a and 1b. FIG. 1a shows a conventional stud 12 fixed to a sole 11 prior to application of the ‘braking force’. FIG. 1b, shows the position of the stud once the braking force is applied; the stud 12 has pivoted about a connection point 13 between the stud 12 and the sole 11. As can be seen, this pivoting causes deformation of the sole, which can cause discomfort to the wearer. Furthermore, the angle of the leading surface 12a of the stud 12, which opposes the braking force, has changed. The surface 12a has tilted substantially, and the effectiveness of the stud to provide traction has therefore decreased.
Conventional studs are usually frusto-conical in shape, tapering towards their distal ends. This tapering increases the studs' ability to penetrate the ground upon ground contact. In general, the smaller the studs, the better they are at ground penetration (at any given penetration force). However, the smaller the studs are, in general, the worse they are at coping with the forces applied to them upon ground contact.
Japanese Patent Application No. JP2002-272506 discloses a stud arrangement in which studs are arranged in clusters. Each cluster has three studs linked by connection elements. The purpose of this arrangement is to reduce the ‘push-up feeling’, i.e. the discomfort caused by forces transmitted from the studs to the sole of the wearer's foot, when the studs contact the ground, since the forces are spread across the studs of the stud cluster, and thus over a wider area.
European patent application No. EP 1234516 discloses a sole structure for a football shoe that is divided into six portions having different rigidities. Sole pressure distribution diagrams are used to determine the appropriate rigidity for each portion. Blade-shaped studs are placed on the sole structure only at areas of high pressure, and the orientation of the blade-shaped studs is based on ‘active direction distribution diagrams’ so as to sustain forces applied from the ground to the foot.